Process for cross-linking high polymers with high energy ionizing irradiation in the presence of nitrous oxide



United States Patent O PROCESS FOR CROSS-LINKING HIGH POLYMER This invention relates to a new and useful method for treating high polymer compounds by an ionizing radiation. More particularly this invention comprises subjecting a high polymer compound to an ionizing radiation to improve the quality of the said high polymer compound.

It is known that exposure of high polymer compounds to an ionizing radiation initiates either cross-linking or decomposition reactions therein. Cross-linking upon such irradiation is characteristic of polyethylene, polystyrene, polypropylene, polyamide, polyester, polyacrylic ester, polyvinyl chloride, natural rubber and synthetic rubber. The decomposition reaction characterizes polymethacrylic ester, polyisobutylene, cellulose and fiuoro resin.

It has been known that air or oxygen, if they are present during the irradiation, can remarkably inhibit the crosslinking reaction, and therefore it has been proposed to eticct such irradiation in a vacuum or in the atmosphere of an inert gas, such as nitrogen, argon, etc., thereby to insure increased effect of crosslinldng. It has also been known that during or after the irradiation, the irradiated high polymer compound may be heated or contacted with a solvent thereby activating the molecular motion of high polymer compound so as to have increased crosslinking eifect. Furthermore, the addition of a crosslinking promotor, for example, thiourea or cresol for polyethylene, or diaryl sebacate or triallyl cyanurate for polyvinyl chloride, also has been proposed.

It is one object of the present invention to provide a method for improving the solvent resistance, heat resistance or other properties of a high polymer compound, especially that of the crosslinking type, which comprises subjecting the high polymer compound to the irradiation of an ionizing radiation in the presence of nitrous oxide (N Another object of the present invention is to provide a method for crosslinking a high polymer compound to the possibly highest degree by the irradiation of an ionizing radiation in an amount as little as possible, while the decompositionof -said high polymer compound is suppressed to the possibly lowest degree.

Other objects, features, capabilities and advantages which are comprehended by the present invention will be apparent from the following specification and claims.

According to the present invention, a high polymer compound is subject to the irradiation of an ionizing radiation ray in the presence of nitrous oxide. In this case, the degree of crosslinking of the high polymer compound can be highly increased as compared with that where the irradiation in the same dose amount is effected in vacuo or in the atmosphere of nitrogen or argon instead of nitrous oxide.

In the prior art where the irradiation is efiected in vacuum or in the atmosphere of an inert gas with the addition of a previously known crosslinking agent, it was found that the said crosslinking agent is eliective to eliminate the crosslinking-inhibiting etiect which is raised by the action of oxygen, but the agent has no substantial crosslinking-promoting effect. In contrast, if the irradiation is efi'ected in the presence of nitrous oxide according o the present invention, such crosslinking agent serves 3,231,481 Patenteci Jan. 25, 1966 not only to eliminate the 'crosslinking-inhibiting effect but also to promote substantial crosslinking effect. As to the reaction mechanism, it can be assumed that nitrous oxide decomposes by the action of radiation ray to form a free radical or ion which activates the neighboring molecule of the high polymer compound to effect the formation of new crosslinking. I

A crosslinking agent which is used heretofore is either liquid or solid, and therefore it must be incorporated into a high polymer compound by means of rollers or the like. In accordance with the present invention, on the other hand, nitrous oxide gas can advantageously diffuse or penetrate with the molecule of a high polymer compound.

High polymer compounds of the crosslinking type, which are usable in a method of the present invention, include all high polymer compounds 'beingcapable'of crosslinking by the action of an ionizing radiation, e.g. polyethylene, polypropylene, polyamide, polyester, polyacrylic ester, polyvinyl chloride, natural rubber and'synthetic rubber. These high polymer compounds may be in any desired shape, such as film, sheet, rod, tube and others.

Ionizing radiation Which includes used herein is gamma rays, X-rays, electron-beams or other by high energy radiation. Suitable exposure dosages may be from 1X 10 roentgens to 1X 10 roentgens.

It may be permissible for the nitrous oxide atmosphere which is employed herein to include a little amount of an inert gas, e.g. nitrogen, argon or the like. However, the presence of a free radical scavenger like oxygen must be avoided to the possibly highest degree. The allowable imit of such free radical scavenger may be less than 10% based on the volume of the total atmosphere.

As to the irradiation conditions, they are not particularly limited. As shown in the following examples, however, higher temperature is preferable because it can accelerate crosslinking reaction, and higher pressure of nitrous oxide also is preferable.

EXAMPLE 1 Low density polyethylene film with a thickness of 0.03 mm. is subjected to the irradiation of Co 'y-ray at 20 C. in the presence of nitrous oxide gas under a pressure of 600 mm. Hg. The degree if crosslinking of this-film is determined by extracting the film with Xylene at C. for 15 hours and measuring the undissolved portion. For reference, swelling ratio is also measured. The experiment is repeated under the same conditions excepting that a vacuum of 10* mm. Hg is used instead of the nitrous oxide atmosphere. The results of both experiments are shown for comparison in the following table.

Table 1 Percent gel Percent Swelling traction I ratio Radiation dosage (rocntgens) Vacuum N 20 Vacuum N20 EXAMPLE 2 Polystyrene film, 0.1 mm. thick, and polystyrene rod, 1 cm. diameter, are subjected to the radiation of Co gamma ray in the presence of nitrous oxide at 20 C. under a pressure of 600 mm. Hg. The degree of crosslinking is determined by measuring undissolved materials after the extraction of said film or rod with boiling benzene for 15 hours. For comparison, the same experi- 2X10 roentgens in the presence of nitrous oxide under a pressure of 600 mm. Hg at various temperatures. The degree of crosslinking is determined by measuring the insoluble part as gel fraction after the extraction of the film with xylene at 80 C. for 15 hours. For comparison, the same experiment is repeated but a vacuum of 4 f; 10* mm. Hg is employed. The results are set forth in the following table.

Table 5 Table 2 5 Percent, Golfraction Temp.

Film Rod Vacuum N20 Radiation dosage (roentgens) Vacuum N Vacuum 6 2 0. 6 69.7 76.3 74.3 82.4 0 0 0 80.1 87.6 47 51 97 83.0 92.2 75 82 97 75 84 98.5

7 EXAMPLE 7 EXAMPLE 3 Low density polyethylene film, 0.3 mm. thick, is ir- Raw rubbert sheet, 1 mm. thick, is irradiated with 2 g??? t ray m 9 dose fg mev. electron-beams in a dose amount of 5 10 roentgms presencefof q i. un i gens at C. The degree of crosslinking is determined g e 2 f i h m by measuring the percentage of insoluble materials after 20 e t h lfif as m i p e e e pras' the extraction with boiling benzene for 15 hours. Nitrous sure t e er cross hung 8 set 15 0 Serve oxide gas pressure is kept at 600 mm. Hg in each case. Table 6 g The results obtained are set forth in the following table. Pressura (N20), Gel fraction, Table 3 mm. Hg: percent 0 74.3

Percent, Gelfraction Vacuum N20 2388 32% Na u bb r 39-2 2%; What we claim is:

3: E 1 1. A process for increasing the cross-li king in a high- A molecular weight polymer capable of being cross-linked EXAMPLE 4 by exposure to high energy ionizing radiation which comprises irradiating the polymer by high-energy ionizing Nylon sheet, 0.085 mm. thick, 15 irradiatsedwith an radiation with a dosage in the range, f f 1x106 to electron-$31115 m dose amount of 2X10 mentgens 1X10 roentgens in the presence of a catalytic amount of and then immersed in para-cresol at 90 C. The change gaseous nitrous oxide catalyst. in length of this sample is determined by measuring the t A process according to claim 1 in which the high iength before and after i lmmersloni l g f 40 molecular weight polymer is a member of the group in vacuum causes elongat on of aboutr2. 4 times its origiconsisting of polyamides, polyesters rubber, polyole nal length, while that in the rutrous oxide atmosphere fines, polystyrene, polyvinyl chloride and polyacryhc (600 mm.'Hg) causes elongation of at most 2.0 times. ester 7 EXAMPLE 5 3.The process according to claim 1 in which the polymer 1s polystyrene. Atactlc Polypropylene mass is lrradlated Wlth C069 4. The process according to claim 1 in which the gamma ray in a dose amount of '7 .0 1O roentgens at Polymer is nylon '20 C. After the irradiation, the said mass is immersed 5 The process according to l i 1 i hi h h l in toluene to determine the percentage of insoluble part met is polyethylehg as gel fraction. Nitrous oxide gas pressure is 600 mm. 6. The process according to claim 1 i which polymer is polypropylene.

Table 4 7. The process according to claim. 1 in which the G61 aCUQH, polymer is rubber. percent Nitrous oxide 5 References Cited by the Examiner "acuum UNITED STATES PATENTS 33:223. \us in Low density polyethylene film, 0.03 mm. thick, 1s 1r- 3 091922 9/1961 Zimm' radiated With gamma ray of C060 in a dos amo of 3:O21:269 2 19 Miner 2O4 154 OTHER REFERENCES Martin: Chemical and Engineering News, April. 4,

r 1955, vol. 33, No. 14, pp. 1424-1428.

MURRAY TILLMAN, Primary Examiner, 

1. A PROCESS FOR INCREASING THE CROSS-LINKING IN A HIGHMOLECULAR WEIGHT POLYMER CAPABLE OF BEING CROSS-LINKED BY EXPOSURE TO HIGH ENERGY IONIZING RADIATION WHICH COMPRISES IRRADIATING THE POLYMER BY HIGH-ENERGY IONIZING RADIATION WITH A DOSAGE IN THE RANGE OF FROM 1X10**6 TO 1X10**9 ROENTGENS IN THE PRESENCE OF A CATALYTIC AMOUNT OF GASEOUS NITROUS OXIDE CATALYST. 